1. Field of the Invention
The present invention relates to a process for continuously producing an alkyl ester of acrylic or methacrylic acid by ester interchange reaction between a starting alkyl ester of acrylic or methacrylic acid and a starting alcohol.
Hereinafter, for clarity and easiness purposes, the starting alkyl ester of acrylic or methacrylic acid may be referred to as "alkyl ester (A)," the starting alcohol may be referred to as "alcohol (B)," the resulting intended alkyl ester of acrylic or methacrylic acid may be referred to as "alkyl ester (C)" and a by-produced alcohol may be referred to as "alcohol (D)."
2. Description of the Related Art
It is known that an alkyl ester of acrylic or methacrylic acid can be produced by ester interchange reaction. The ester interchange reaction is an equilibrium reaction and various catalysts have been proposed to increase the reaction rate. Examples of the proposed catalysts include acids such as sulfuric acid and p-toluenesulfonic acid (Japanese Patent Application Laid-Open No. 62-42948), basic substances such as alcoholates, hydroxides, carbonates, phosphates, oxides and complexes of alkali metals and alkaline earth metals (Japanese Patent Application Laid-Open Nos. 53-144523, 55-27118, 56-77242, 57-93930, 62-185051, 63-5054, 63-5055, 2-104559, 2-193944 and 6-293702), metal alcoholates such as aluminum alcoholates and magnesium alcoholates (Japanese Patent Application Laid-Open No. 50-19716), titanium compounds such as titanium alcoholates, titanium phenoxide and alkyltitaniums (Ger Offene 2319688, U.S. Pat. No. 3,686,268 and Japanese Patent Application Laid-Open No. 1-258642) and lead compounds, zinc compounds, tin compounds and complexes of metals such as copper, iron and zirconium (Japanese Patent Application Laid-Open Nos. 53-141213 and 53-105417 and U.S. Pat. No. 4,202,990). Further, a process using a metal or a metal oxide as the catalyst has been proposed (Japanese Patent Application Laid-Open No. 52-111512).
In the proposed process, a reaction apparatus having a distillation column, a fractionating column, etc. at an upper position of a reaction vessel is preferably used so that a lower alcohol by-produced in a batch process is separated from the remaining starting mixture, the reaction product and the solvent by distillation. It is apparent that the ester interchange reaction takes place only in a reaction vessel containing a catalyst. The distillation column, etc. disposed at an upper position of the reaction vessel is used only to separate the lower alcohol by-produced in the ester interchange reaction from the other components in the reaction vessel. Therefore, in the proposed consecutive process of the reaction and distillation, the ester interchange reaction and the distillation are conducted at the separate portions in the reaction apparatus. Namely, in the distillation column, only the distillation is carried out and the ester interchange reaction does not occur therein. In such a process, the ester interchange reaction, which is conducted in the liquid phase in the reaction vessel, proceeds only when the equilibrium of the reaction is shifted toward the product side by the moving of the by-produced low-boiling alcohol from liquid phase to vapor phase through the vapor-liquid interface. Since the reaction vessel used in the conventional process is a tank having a vapor-liquid interface area as small as the cross-sectional area of the tank, it is known that the ester interchange reaction proceeds very slowly. For example, the batch process takes a reaction time as long as 2 to 5 hours to obtain a sufficient result. The conventional process further requires a high reaction temperature to obtain a sufficient reaction rate. The high reaction temperature occasionally decreases the selectivity of the intended alkyl ester (C) due to disadvantageous side reactions such as the polymerization of the starting alkyl ester (A) or the resulting alkyl ester (C) and the addition reaction of the alcohol to the double bond of the alkyl esters (A) and (C).
In the conventional process conducting the ester interchange reaction and the distillation at the separate portions in the apparatus, the ester interchange reaction is carried out in a batch process by placing the alkyl ester (A), the alcohol (B) and a catalyst in a reaction vessel, and then, allowing the ester interchange reaction to take place. Thus, no continuous process, in which the starting materials are supplied continuously and the products are taken out continuously, has been proposed in the production of an alkyl ester of acrylic or methacrylic acid.